The present invention relates to a power supply system for a computer monitor, and more particularly to a power supply system for both 110 V and 220 V AC sources which eliminates harmonics efficiently.
The ideal AC source voltage waveform is a pure 60 Hz sine wave. If a sine wave has a distortion (as shown in FIG. 1), it indicates the presence of a harmonic. The harmonic can be 120 Hz, 180 Hz, 240 Hz, . . . , that is, beyond the 60 Hz fundamental frequency.
When the 60 Hz square wave shown in FIG. 2a is analyzed by Fourier Series, the waveform shown in FIG. 2b results. FIGS. 4a to 4d show the input characteristics of a computer monitor power supply circuit in which the electric current waveform (see FIG. 4d) greatly differs from the sine wave, indicating the presence of significant harmonic current. Except for resistance type electrical heating apparatus, the input current of most electronic apparatus, and particularly ones using a switching power supply or phase control, carries surges. If the use of these electrical apparatus reaches a certain amount, as shown in FIG. 3, the harmonic current malfunctions shown in the following Table I will occur in the whole power supply system when the total load current of the end users contains a certain amount of harmonics.
TABLE I __________________________________________________________________________ Interfered Apparatus Malfunction Phenomenon __________________________________________________________________________ Power System Series reactance burnout overheat, vibration, noise due to overcurrent Fuse broken melted due to overcurrent Motor circuit breaker false action Electric home appliance Audio equipment noise Television set picture jumping Others Motor noise, vibration Elevator vibration, shut down Control Circuit false action Harmonic filter trip off due to overcurrent __________________________________________________________________________
Therefore, the IEC issued standard number IEC-1000-3-2 in 1993 to define harmonic current levels in public low voltage power supply systems. Monitors are classified as D type electric appliances, and the harmonic current of monitors is defined as shown in the following Table II..
TABLE II ______________________________________ Maximum allowable Maximum allowable harmonic current harmonic Oder of harmonic per Watt current (n) (mA/W) (A) ______________________________________ 3 3.4 2.30 5 1.9 1.14 7 1.0 0.77 9 0.5 0.40 11 0.35 0.33 13 .ltoreq. n .ltoreq. 39 3.85/n 0.15 .times. 15/n (odd oder harmonic effective) ______________________________________
In the conventional monitor power supply circuit shown in FIG. 4a, the load of the monitor must be capacitive because the rectified output is directly connected to the filter capacitor, and therefore an inductor is needed for neutralization to make the load close to pure resistance, so as to reduce harmonic current. The methods shown in the following Table III are commonly adopted.
TABLE III __________________________________________________________________________ Method Item 1. Series Inductor 2. Converter 3. Active Filter __________________________________________________________________________ harmonic improvement barely acceptable good excellent Cost low 2.5 times above method 3.5 times above method 1 1 Efficiency 90 .about. 95% 80 .about. 85% 80 .about. 85% Power Factor 0.65 .about. 0.75 0.8 .about. 0.9 0.9 .about. 0.99 Interference prevention not needed difficult to overcome difficult to overcome 88 V .about. 264 V input not allowed barely allowed allowed Control circuit FIG. 5a FIG. 5b FIG. 5c __________________________________________________________________________
The aforesaid second method and third method have the drawbacks of high cost and serious electromagnetic interference, and therefore they are not popularly accepted. The aforesaid first method costs less, but is not suitable for a power range of from 110 V grade (110 V.+-.20%) to 220 V grade (220 V.+-.20%), and therefore the inductor used in a monitor to eliminate harmonics must be relatively changed subject to local power voltage. This limitation causes a great trouble to manufacturers and distributors. In order to facilitate the manufacturing process and sales management and to prevent a wrong use, a harmonic eliminating device should be applicable to both 110 V grade and 220 V grade power supplies, i.e., applicable to the currents ranging from range within AC 80 V.about.AC 264 V.